1. Fields of the Invention
The present invention relates to a bus-based optical network system, especially to an optical network system in which the medium access of the network meets the requirement of “ideal fair behavior”. The ideal fair behavior indicates that the medium access of the optical network units (ONUs) is not affected by the network topology as well as the position of ONUs. Moreover, the number of central offices (COs) and the amount of optical fibers are both small so that the construction cost and maintenance cost can be dramatically reduced.
2. Descriptions of Related Art
In recent years, the required index of bandwidth for public access networks is increased due to bandwidth requirements for various communication services. In conventional access networks, the provided bandwidth and transmissible distance are restricted by the characteristics of twisted-pairs. Thus network operators are dedicated to promoting the implementation and usage of FTTH (fiber-to-the-home) access networks. And the FTTH access networks are growing in several areas. Yet the high installation and maintenance cost is a barrier for globalization of FTTH networks. Thus how to reduce installation and maintenance cost while the communication quality can be assured is an important issue for the promotion of the FTTH access networks.
The public access network can be considered as a bridge between the backbone network and its users. A wider bridge can carry higher traffic. The capacity of optical fibers is much larger than that of twisted pairs. The bandwidth which a single user requires is so little that the capacity of an optical fiber can be shared by a plurality of users. Thus in the establishment of FTTH networks, to reduce cost by sharing media is feasible. In order to provide a medium-sharing environment, a plurality of multiple-access structures is proposed for constructing FTTH networks. These multiple-access structures can be divided into two groups according to the number of optical network units (ONUs) connected with a set of optical fibers. In the first group, each set of optical fibers is connected with an ONU while each set of optical fibers is connected with several ONUs in the second group. The network topology of the second group is a ring, as shown in FIG. 2, the end of the ring fibers is connected with an optical line terminal (OLT) situated at a central office (CO). The OLT includes two transmitters and two receivers. One set of the transmitter and the receiver generates upstream slots while the other set generates downstream slots. Directions of the upstream and the downstream slot flows are opposite to each other. On the other hand, the first group is further divided into two subgroups, i.e. a star topology and a tree topology as shown in FIGS. 3 and 4 respectively, according to the existence of splitters or passive fiber branching points. In the star topology, there is one central node, i.e. a CO, connected with ONUs by radial optical fibers. In the tree topology, splitters or passive fiber branching points are sub-centers and optical fibers branch out from the sub-centers to connect with ONUs. Moreover, there is an optical-fiber link between the OLT and the sub-center. The link comprises upstream and downstream channels.
The FTTH network of the first group requires large amount of optical fibers due to low bandwidth utilization efficiency. Thus high cost is required to install the FTTH network of the first group. As to the ring FTTH network, its bandwidth utilization efficiency is so high that the required amount of optical fibers is much less than star or tree FTTH networks. Thus the constructional cost paid for optical fibers can be reduced. However, in the ring FTTH network, the distance between a CO and its farthest ONU is shorter than that of star or tree FTTH networks. This leads to increase the number of COs. The more the number of COs, the larger the cost paid for constructing COs. From the perspective of constructional cost to compare ring topology with star or tree topology, the reduced cost due to fewer optical fibers for ring topology is used to compensate the increased cost according to the addition of COs. On the other hand, the cost due to fewer COs for star or tree topology is used to compensate the additional cost according to the largely increased amount of optical fibers. Furthermore, the maintenance cost of the FTTH network is related to the amount of optical fibers. The FTTH network with large amount of optical fibers not only complicates the distribution of optical fibers but also increases the difficulty in optical-fiber management and maintenance. This results in higher FTTH maintenance cost. Thus if both the number of COs and the number of optical fibers can be simultaneously reduced, not only the FTTH construction cost but also the FTTH maintenance cost will be lowered.
In most FTTH medium-sharing environments, time-division multiple access (TDMA) is used on the upstream channel while time-division multiplexing (TDM) is used on the downstream channel. In the downstream channel, besides a few control and maintenance messages from OLTs, most messages are the responses to communicative services initiated by network users. Although TDM can distribute bandwidth evenly, the channel capacity is not utilized efficiently. Moreover, the traffic responded to services initiated by a network user varies with the services. Yet the TDM technique is unable to distribute bandwidth according to various services. As to the upstream channel, all ONUs connected with an OLT are senders. The TDMA system provides the bandwidth requested by customer premise equipments (CPEs) attached to ONUs. Under TDMA operations, if some ONUs initiate when the network is with heavy load, these ONUs are unable to get the requested bandwidth in time and need to wait for a period. Otherwise, in order to reduce constructional cost, the number of ONUs must be increased. As the number of ONUs is increased, the duration of heavy-load state will be extended. Thus the period that an ONU waits for requested bandwidth will also be extended.
On the other hand, a mechanism, such as usage parameter control (UPC) that executes control functions including connection admission control, resource management, and priority control etc. on the backbone network, rejects partial traffic of an ONU which exceeds the maximum traffic agreed by commercial contract. This leads to the retransmission of the rejected traffic between the backbone network and the ONU. The continuously repeated retransmission occupies a certain amount of bandwidth on the upstream channel. The useless occupation not only reduces the throughput of ONUs but also increases heavy-load duration. Under the vicious circle, the waiting time of messages in ONUs will be lengthened. However, in order to reduce costs, the number of ONUs connected with an OLT must be increased. Hence, the heavy-load state will occur at high frequency and the heavy-load duration keeps increasing. Thus to reduce constructional cost will result in the dramatic degradation of the network performance. The performance represents the average waiting time of an ONU. The average waiting time of an ONU is the average duration that data segments wait in the first buffer of the ONU queue for an available slot on bus.
Refer to U.S. Pat. No. 6,504,853, the basic topology of a DTM network is a bus connecting all nodes but can also by realized by any other kind of structure, e.g., a ring structure (Row 56-57 Column 1). DTM is a circuit switched network and intended to be used in public networks as well as in local area networks (LAN's) (Row 26-28 Column 1). This patent mainly uses a technique provided for reallocating slots used for transferring data between nodes (Abstract). This prior art relates to a transmission system, not a transmission system. The network is suitable for short distance and small area (such as CO). A lot of nodes are densely deployed in the network for exchange. Thus while designing the network structure and the components used, problems of long distance network such as optical dispersion and energy loss are not considered. On the other hand, FTTH access network is used within a large area such as a city, a county, a town, etc. It reaches each node with the area, such as each users' home. That means nodes are connected from house to house. The messages received and transmitted by each node are transmitted by the FTTH access network but no message exchange occurs between nodes. Thus while dealing with the design of the network structure and the components used, problems of long distance network such as optical dispersion and energy loss should be considered here. In order to prevent interference caused by reflection of the signals and improve quality of signals received, slot terminators are used to avoid reflection of optical signals. In the U.S. Pat. No. 6,504,853, no slot terminator is mentioned. Thus it is unable to be applied to FTTH access network.
In the Row 15 Column 2, it is mentioned that the majority of the slots in a cycle are data slots. Access to data slots changes over time, according to traffic demands. Write access to slots is controlled by slot access, sometimes referred to as slot tokens. A node controller may write data into a specific slot only if the node has write access to this specific slot. The slot access protocol, or token protocol, guarantees the slot access to be conflict free, which means that several nodes do not write into the same slot.
The network uses slot access protocol such as token protocol to ensure no conflict of the slot access. Thus the network is a token network, not a TDMA network. Refer to R. M. Newman, and J. L. Hullet, “Distributed queueing: A fast and efficient packet access protocol for QPSX,” in New Communication Services: A Challenge to Computer Technology, P. Kiihn (Ed.), North-Holland, pp. 294-299, 1986, it is learned that packet delay of the token network is more often than that of the TDMA network. This means the performance of the network in the U.S. Pat. No. 6,504,853 is much lower than the TDMA network.
Moreover, refer to the Row 33 Column 3 of the U.S. Pat. No. 6,504,853, the transfer of slot ownership is initiated by a request from the node having a lack of slot capacity. Such a request, or any other type of reallocation initiation, may for example be provided when the node has borrowed slots a predefined number of times, when the ratio between the number of slots having the node as temporary home node and the number of slots having the node as primary home node exceeds a predefined value, when the allocation or “loan” has to be directed over a substantial distance over the network. As another alternative, a predefined fraction or a predefined number of slots may be arranged to change owner.
In U.S. Pat. No. 6,504,853, “borrow and loan” initiated by various requests is used to regulate write access to time slots between system nodes. Thus data in the node of the bus-based optical network system is unable to be written into another empty time slot directly according to business contract engaged between network managers and clients. The execution of each node can't satisfy requirements of contents of the business contract.
Refer to U.S. Pat. No. 4,630,256, a bidirectional dual network is revealed. In accordance with the invention, two channels operated in opposite directions are established on one optical fiber. The network is constructed based on Wavelength Division Multiplexing (WDM). Thus the bandwidth of each sub-network is reduced and the speed is decreased, much more lower than the system having only one channel per one optical fiber. The quality of optical signals is also getting worse and the propagation distance is shortened. Thus the network is applied to small area such as private network in a building or between adjacent buildings. From the point of view of the data transmission, the network is neither applicable to the medium and long distance communication (such as network across the city or town), nor to the public networks because that construction and maintenance cost of FTTH network is affect by distribution range of the network lines.
Refer to U.S. Pat. No. 7,639,694, a centralized control network is used in the system. Due to centralized control and distribution of access of slots of the TDMA system, the centralized control mechanism assigns the number of slots allowed and positions of these slots to the node that delivered a request for sending messages. Thus while the node in the centralized control system sending messages, it first sends a “sending request message” and then waits for “permitting to send messages” (messages that permit the node to send messages). The content of the permitting to send messages includes the number of slots allowed and positions of the slots. The “sending request message” and the “permitting to send messages” are collectively called control messages. The control messages are transmitted by specific control slots. That means the control messages share a part of bandwidth.